Nowadays, when you go to a movie theater, the protocol is rather simple and clear: You buy a ticket prior to the start of the movie, you enter the theater, and when the movie ends, you have to leave. (Most theaters won't sell you a ticket for a showing that has already started, although it depends on the country and the theater. In many theaters you are allowed to enter the showing in the middle of it, if you have a valid ticket, but also this varies. Many theaters stop accepting people after a certain time after the movie has started.)
This makes sense, and it has been so for a pretty long time. However, it hasn't always been so.
You see, prior to about the 1960's, at least in the United States (and many other places) most movie theaters operated quite differently. It wasn't a business model (nor a moviegoing culture) of "you pay to see one movie, you enter when the movie starts, you get to watch it, then you have to leave". No, it was a business model of "you pay for admittance, you can enter and leave whenever you like, and watch whatever happens to be on the screen at the time".
It was, in fact, surprisingly common practice for people to just go to a movie theater completely ignoring movie starting times, and just enter and watch the movie, even if it was in the middle of it. Then, after the movie was over, they just stayed and watched the first half of it, before leaving.
From a certain perspective this made some sense. You see, this was before the widespread (and even existence) of the TV, and movie theaters were (besides newspapers) the most common way of watching the news and visual entertainment. Newsreels were shown between the movies, and these were popular.
It still feels a strange practice, though. Watch the latter half of a movie, then some newsreel, then the first half of the movie, and leave. But that was quite common in the pre-60's America (and probably elsewhere).
You might have heard that when Alfred Hitchcock made his movie Psycho in 1960, he demanded that no people be admitted into the theater after the movie had started. It was precisely because of this common practice. It was also one of the most influential events that changed the practice to what it is nowadays.